A line segment has endpoints A=(1,3) and B=(13,19).
a) Find the equation of the circle for which the segment AB is a diameter.
Ans: (x - 7)^2 + (y - 11)^2 = 100
b) Accurate to the nearest tenth, the circumference of this circle is _______ ?

I was able to find a)the equation but I don't know how to find the circumference, PLEASE HELP!!